1. Field of Invention
The present invention relates to a photolithographic processing method. More particularly, the present invention relates to a method of producing a phase shifting mask (PSM).
2. Description of Related Art
As the level of device integration increases, high-resolution masks in photolithographic processing are needed in the production of semiconductor devices having the dimensions required in semiconductor fabrication. One method of increasing the resolution of masks used in photolithographic processing is to use a light source having a shorter wavelength. For example, deep ultra-violet light, having a wavelength of 2480 .ANG., can be generated by a krypton fluoride laser and used as a light source for photolithographic exposure. Although shortening the wavelength does increase the resolution of photolithographic processing, the depth of focus (DOF) of the devices may be insufficient, thus compromising the quality of products. Another method of increasing, resolution, actively being researched by semiconductor manufacturers, is the use of a phase shifting mask.
In principal, a phase shifting mask is a conventional photomask with an added phase shifting layer. Through positive and negative interference generated by the phase shifting layer during light exposure, an image pattern projected onto the wafer by the light projector will have a higher resolution. The biggest advantage of using the phase shifting mask is its capacity for increasing the resolution of the projector without having to use a new light source. This means that the modification is entirely in the photomask. Hence, a higher level of device integration can be achieved with the existing projector equipment.
FIGS. 1A, 2A and 3A are cross-sectional views showing three types of conventional phase shifting masks (PSMs). FIG. 1A is a cross-sectional view showing one type of conventional alternating PSM. The alternating PSM is suitable for producing line patterns for semiconductor devices on a wafer. The alternating PSM has a transparent substrate 100 made of quartz and has a chromium layer plated on top of the transparent substrate 100 that acts as a masking layer 102. Moreover, a phase shifting layer 104 made of molybdenum silicon oxynitride (MoSi.sub.Z O.sub.X N.sub.Y), which generates a phase shift angle of 180.degree. in incoming light, is formed above the transparent substrate 100 as well. Since the phase shifting layer 104 is placed in alternating positions, light shining on the phase shifting mask generates phase shift angles of 0.degree. and 180.degree. alternately. Therefore, light intensity resulting from diffraction is cancelled, producing a zero-point light magnitude and increasing photolithographic resolution.
FIG. 2A is a cross-sectional view showing a conventional half-tone phase shift mask (HTPSM). Normally, a HTPSM, which has a larger depth of focus, is used to produce hole patterns for semiconductor devices on a wafer. The light transparency of this photomask is roughly between 3-10%. A phase shifting layer 204, which can generate a 180.degree. phase shift in incoming light, can be formed on portions of a transparent substrate 200 so that hole patterns can be formed on the exposed transparent substrate area 206. The HTPSM operates by shifting incoming light through phase angles of 20.degree. and 180.degree. alternately. Consequently, the magnitude of light at the junction of the hole pattern 206 and the phase shift layer 204 cancel each other, producing zero-point light magnitude. Hence, both contrast and resolution during light exposure can be increased.
FIG. 3A is a cross-sectional view showing a conventional rim phase shifting mask. The rim phase shifting mask is formed by depositing a phase shifting layer 304 over a transparent substrate 300 outside the region where the desired semiconductor device patterns, area 306, are formed. Through the generation of a zero point light magnitude due to interference, the resolution of photolithographic processing can be increased. As shown in FIG. 3A, the phase shifting layer 304 is sandwiched between a masking layer 302 and the transparent substrate 300. Furthermore, the masking layer 302 covers only a portion of the underlying phase shifting layer 304 while exposing a strip of phase shifting layer 304 above the transparent substrate 300 in order to increase the resolution.
However, all of these three phase shift masks face the same types of problems. Due to difficulties in controlling the etching conditions, the transparent substrate (quartz) is often damaged when the masking layer or the phase shifting layer is patterned, thereby forming defects as shown in region 150 in FIGS. 1A, 2A and 3A. Thickness of material is one of the critical factors that can seriously affect the phase shift angle of the photomask. At present, repair stations still do not have the capacity to repair the defective regions 150 by depositing a filler layer. A repair station can at most etch away a portion of the material in the defective region 150 so that phase shift errors in that portion can be removed.
In the conventional etching-repair method, the damaged transparent substrate is etched to a certain depth. By adjusting the depth of etch, the etched transparent substrate region is able to maintain the same relative phase angle difference as its adjacent pattern. For example, FIGS. 1B, 2B and 3B show the formerly defective regions 150 in areas 106, 206 and 306 of FIGS. 1A, 2A and 3A, respectively etched to form trenches 180 having a 360.degree. phase angle shift. However, due to difficulties in controlling the etching rate and the in-line monitoring facilities, etching a trench to a depth that can produce a 360.degree. phase shift angle is difficult. In the worst cases, the defective region 150 may be damaged beyond repair and the whole photomask then has to be thrown away.
In light of the foregoing, there is a need to provide an improved method of producing phase shifting masks.